This application is based on and claims the priority under 35 U.S.C. xc2xa7119 of German Patent Application 101 51 780.7, filed on Oct. 19, 2001, the entire disclosure of which is incorporated herein by reference.
The invention relates to a method and an apparatus for adjusting the thread braking force of a weft thread brake arranged between a weft thread supply system and a weft thread reserve store or accumulator.
In shuttleless looms, the weft thread is pulled from a stationary weft thread supply system, which comprises, for example, one or more weft thread supply spools, also called bobbins or pirns. Due to the high unwinding or drawing-off speed of the weft thread during the weft insertion in a gripper rapier loom or a gripper shuttle loom, and due to the force-transmitting weft insertion in an air jet or water jet loom, it is generally known to provide a weft thread reserve store or accumulator, also known as a weft storage or pre-winding device, between the supply spools and the weft insertion system. The weft thread accumulator serves to accumulate and hold ready a required length of the weft thread for carrying out the next weft insertion. In this regard, the required length of weft thread is pre-wound onto a drum of the weft thread accumulator, and then during the weft insertion, the required thread length is drawn off from the reserve winding of thread on the drum. The thread length is defined by the number of windings and the drum diameter. When the weft thread being woven has an elastic character, the winding tension of the thread on the drum of the thread accumulator can have a variable and possibly significant influence on the true thread length that is to be inserted into the loom shed. For this reason, it becomes important to control the winding tension of the thread on the drum of the thread accumulator.
The unwinding or drawing-off behavior of the thread from a thread supply spool is variable. For example, for a prescribed unwinding or drawing-off speed, the thread tension will be lower for a relatively full spool, and will be higher for a relatively empty spool (i.e. a spool with only a small amount of remaining thread supply thereon). In order to try to avoid the disadvantageous thread tension variations in this context, and the resulting differing thread lengths in the weft thread accumulator, it is known to use uncontrolled or unregulated thread brake systems on the thread supply path. These known thread brakes are adjusted to a relatively high braking tension, i.e. are adjusted relatively strongly to a braking operation, to ensure that a sufficiently high thread tension is achieved under all operating conditions. The disadvantage of such known uncontrolled thread brakes is that the thread tension achieved thereby is thus generally higher than necessary, and higher than the maximum natural thread tension that arises during the unwinding of the thread supply spool. That in turn means that the thread is loaded, or stressed and strained, more than would be necessary. This causes weak spots in the thread, which lead to thread breaks during the weft insertion. To avoid this, many weavers attempt to operate the loom without a thread inlet or supply brake. Thereby, the weft thread is handled more gently, but unfortunately, the supplied thread lengths exhibit variations that lead to an increase in the weft thread trimming waste.
In view of the above, it is an object of the invention to provide a thread brake in the area between the thread supply spool and the thread accumulator or storage device, while reducing the mechanical loading of the weft thread caused by the weft thread brake system in this area, and while also achieving a substantially constant winding tension in the weft thread accumulator in order to achieve uniform accumulated thread lengths and a small weft thread waste. The invention further aims to maintain a substantially constant thread supply tension through a thread supply spool change from an empty supply spool to a new full supply spool. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present specification. The attainment of these objects is, however, not a required limitation of the present invention.
The above objects have been achieved according to the invention in an apparatus for variably braking the weft thread between a supply spool and a thread accumulator in a loom, comprising a thread supply system including at least one thread supply spool, a thread accumulator or thread storage device, a thread brake arranged on a weft thread supply path between the thread supply system and the thread accumulator, a weft thread tension sensor arranged on the weft thread supply path between the weft thread brake and the thread accumulator, and a control arrangement or controller that is connected with the thread tension sensor and with an actuator of the thread brake by respective signal transmission links. The controller receives a thread tension signal representing an actual measured value of the thread tension from the thread tension sensor, compares this actual measured value to a nominal, rated or desired value of the thread tension, and then generates a control signal dependent on any difference between the actual value and the desired or rated value of the thread tension. The control signal is supplied to the actuator of the thread brake, so as to control the actuation of the thread brake. Thereby, the weft thread tension is controlled in real time on a continuous or ongoing basis, to be maintained substantially at the rated or desired tension level. As a result, the weft thread is uniformly would onto the drum of the thread accumulator, whereby a uniform pre-winding tension and a uniform accumulated thread length for each successive weft insertion are achieved.
Preferably, the weft thread supply system includes more than one supply spool, and the inventive apparatus further comprises a spool transfer or change sensor, which senses the switch-over from an empty supply spool to a new full supply spool. The signal provided by this sensor, which indicates the change or transfer from the empty spool to the next full spool, is provided to the control arrangement. In response to this signal, the control arrangement provides a suitable control signal to the actuator of the thread brake, which sets the brake to an adjustable or selectable starting brake setting, to achieve a coarse adjustment of the braking force during the time of the thread change from one spool to the next. Thereafter, the fine tension adjustment and closed loop regulation of the thread tension is once again carried out based on the actual measured thread tension as determined by the thread tension sensor, as described above.
The above objects have further been achieved according to the invention in a method of variably braking the weft thread between the supply spool and the thread accumulator. The inventive method involves carrying out the method steps as described above in connection with the operation of the inventive apparatus.
According to the invention, using the described braking system, the required braking force can be continuously controlled or regulated in a closed loop and self-regulating fashion, dependent on the actual existing thread tension as measured with a thread tension sensor. By further providing the spool change sensor, the resulting spool change signal allows the system to switch rapidly, or even preemptively, to a new initial setting of the adjustable braking for the different tension conditions that will prevail with the new full spool. Particularly, a substantial change of the natural supply tension is expected in connection with a change from an empty spool to a new full spool. The spool change sensor gives an advance notice of this expected tension change so that the thread braking system can be accordingly adjusted in a rapid manner. For example, the thread brake can be set to a nominal starting brake setting for the thread spool change period. Immediately thereafter, the normal regulation again continues the fine adjustment of the braking force in a self-regulating manner.
Thereby, according to the invention, it is ensured that the weft thread is only braked to such an extent, at all times, as is necessary to achieve a constant winding tension on the drum of the thread accumulator or storage device. For example, that means that the thread brake will be adjusted to apply a greater braking effect while the weft thread is being drawn off from a relatively fully supply spool, while the thread brake will be adjusted to a nearly open condition, i.e. applying essentially no braking effect, when the weft thread is being drawn off from a supply spool that is nearly empty. In this manner, according to the invention, the weft thread is braked and thus tensioned only exactly as much as necessary (or no more than necessary), and therefore the thread loading is optimized (i.e. unnecessary strains and stresses on the thread are avoided), and the winding tension of the thread on the drum of the thread accumulator is maintained substantially constant. Thus, the thread waste length can be minimized and maintained essentially constant.
According to the invention, basically any type of thread brake can be used, as long as the thread brake can apply a variable controllable tension or braking force and effect. Preferably according to the invention, the thread brake is embodied as a leaf spring brake or as a thread looping or wrapping brake.